int sd_uid_get_state(uid_t uid, char **state);
int sd_uid_is_on_seat(uid_t uid, int require_active, const char *seat);
int sd_uid_get_sessions(uid_t uid, int require_active, char ***sessions);
int sd_uid_get_seats(uid_t uid, int require_active, char ***seats);
int sd_uid_get_display(uid_t uid, char **session);
sd_uid_is_on_seat() may be used to determine whether a specific user is logged in or active on a specific seat. Accepts a Unix user identifier and a seat identifier string as parameters. The require_active parameter is a boolean value. If non-zero (true), this function will test if the user is active (i.e. has a session that is in the foreground and accepting user input) on the specified seat, otherwise (false) only if the user is logged in (and possibly inactive) on the specified seat.
sd_uid_get_sessions() may be used to determine the current sessions of the specified user. Accepts a Unix user identifier as parameter. The require_active parameter controls whether the returned list shall consist of only those sessions where the user is currently active (> 0), where the user is currently online but possibly inactive (= 0), or logged in at all but possibly closing the session (< 0). The call returns a NULL terminated string array of session identifiers in sessions which needs to be freed by the caller with the libc free(3) call after use, including all the strings referenced. If the string array parameter is passed as NULL, the array will not be filled in, but the return code still indicates the number of current sessions. Note that instead of an empty array NULL may be returned and should be considered equivalent to an empty array.
Similarly, sd_uid_get_seats() may be used to determine the list of seats on which the user currently has sessions. Similar semantics apply, however note that the user may have multiple sessions on the same seat as well as sessions with no attached seat and hence the number of entries in the returned array may differ from the one returned by sd_uid_get_sessions().
sd_uid_get_display() returns the name of the "primary" session of a user. If the user has graphical sessions, it will be the oldest graphical session. Otherwise, it will be the oldest open session.